What is non martensitic structure

What is non martensitic structure?

Non martensitic structure is a common quality defect in surface carburizing (carbonitriding). It can reduce the surface hardness, wear resistance and fatigue properties of carburized layer, which has been perplexing the relevant heat treatment workers. In recent years, with the improvement of national machinery manufacturing level, the requirements for non martensitic structure are higher and higher. A lot of application research and engineering application experience have been accumulated in China, but the basic research on non martensitic structure is seriously lagging behind. It is suggested that the relevant material researchers should strengthen the research on the nature and formation of non martensite structure, so as to provide theoretical support for the development of heat treatment equipment and the selection of process methods.
Many problems related to material heat treatment are systematic problems, which can not be effectively solved only by heat treatment process itself. It is necessary to carry out systematic control together with metallurgy, forging, pretreatment, machining and other processes, such as deformation, cracking, insufficient mechanical properties, non Ma and other common heat treatment problems. This paper puts forward some understanding of non martensitic structure, hoping to provide some help to the industry workers.

General situation of non martensitic structure

The ideal structure of carburized (carbonitriding) parts after quenching should be fine acicular high carbon martensite. However, due to many uncontrollable factors, some non martensitic mixed structures such as bainite and troostite (pearlite type) are formed on the surface of gears, which are called non martensitic structure, and traditionally called “non martensitic” structure. If the depth of non martensitic structure exceeds the standard seriously, it is reflected in the mechanical properties that the surface hardness of parts is low, which affects the hardness gradient.
The metallographic observation method of “non equine” structure is usually no corrosion or light corrosion, but there is no clear specification for specific operation method in China“ The “non equine” structure is usually attributed to internal oxidation during carburizing. Figure 1 shows the typical intergranular oxidation of carburized gears.

According to the energy spectrum analysis of scanning electron microscope, the products of internal oxides are Cr, Mn, Ti, Si and other alloy oxides, as shown in Figure 2. The existence of internal oxidation makes the alloy elements near the grain boundary depleted, and the non martensitic structure is easy to appear on both sides of the grain boundary. The formation of internal oxidation and the depletion of alloying elements reduce the stability of retained austenite nearby, and it is easy to decompose during subsequent quenching and cooling, forming a mixture of troostite and bainite.

There are many morphologies of non martensitic structure, which can be simply divided into three categories: black spot, black net and black belt. The formation mechanism of the three different morphologies should be slightly different, and the underlying reason needs to be further confirmed. Among them, black spots are common in carbonitriding process, which may be caused by the high nitrogen potential of furnace gas at the initial stage of carbonitriding, the high nitrogen content in the layer, and the increase of carbon concentration when the carbonitriding time is long, resulting in the decomposition and denitrification of nitrogen compounds, and the formation of holes due to the transformation of atomic nitrogen into molecular nitrogen. At present, some people in China have come to the conclusion that there is a certain transformation order in the formation of non martensitic structure: ferrite block → ferrite block + primary troostite net (the first black net) → primary troostite net (black belt) → primary troostite Net + martensite (the second black net) → martensite + bainite. The author thinks that this conclusion is questionable and lacks the support of theory and evidence.

Introduction of Ellingham oxygen potential diagram

As a kind of chemical heat treatment, carburizing (carbonitriding) is a physical and chemical process. It uses the chemical reaction of atmosphere and physical method to change the chemical composition and structure of workpiece surface, so as to obtain high hardness and high wear resistance. In fact, the whole process can also be simply understood as a physical metallurgical process under solid state conditions. Therefore, the author thinks that the mature theories of metallurgical physical chemistry are also applicable to chemical thermodynamics. As a basic tool of metallurgical physical chemistry, Ellingham oxygen potential diagram can provide some reference for the basic research of chemical heat treatment.
Most of the elements in nature exist in the form of oxides, sulfides or other compounds. In order to intuitively analyze and consider the oxygen affinity of various elements, understand the relationship between oxidation and reduction of different elements, and compare the stability order of various oxides, Ellingham used to generate Gibbs free energy △ FG θ The change of standard Gibbs free energy △ RG of the reaction of elements with 1mol oxygen θ(J·mol-1). Immediate response:

△_rG^θ The binomial relationship with temperature T is plotted. This diagram is also called oxygen potential diagram, as shown in Figure 3.

The position of the straight line in Ellingham oxygen potential diagram can explain the following problems:

• 1) The lower the position is, the easier the reaction is, the more stable the oxide is, and the more difficult it is to be reduced by other elements.
• 2) At the same temperature, if several elements meet with oxygen at the same time, the element with lower position is oxidized first. For example, at 1673k, when Elements Si, Mn, CA, Al and Mg meet with oxygen at the same time, the metal CA is oxidized first, followed by Mg, Al, Si and Mn.
• 3) The low position element can reduce the high position oxide in the standard state. For example, at 1600 ℃, Mg can reduce SiO2 to obtain liquid silicon.
• 4) Because the slope of the generated CO line is different from that of other lines, the CO line divides the graph into three regions. In the region above the CO line, oxides such as Fe, W, P, Mo, Sn, Ni, Co, as and Cu can be reduced by C; Below the CO line, oxides such as Al, Ba, Mg, Ca and rare earth elements cannot be reduced by C; In the middle region, CO lines intersect with other lines, such as Cr, Nb, Mn, V, B, Si, Ti oxide lines. When the temperature is higher than the intersection temperature, element C is oxidized. When the temperature is lower than the intersection temperature, other elements are oxidized. This plays a very important role in the metallurgical process. From the oxidation point of view, the intersection temperature is called the oxidation transformation temperature of carbon and intersecting elements; From the point of view of reduction, it is called the lowest reduction temperature of the element oxide.

Proposal of “selective oxidation”

In the common carburizing (carbonitriding) atmosphere, there are some oxidation components such as CO2, H2O, O2, that is, oxygen pressure exists in the atmosphere, and some alloy elements in steel matrix which are easy to be oxidized will be oxidized first. Different alloy elements have different priorities in the sequence, which can be understood as “selective oxidation”.
The temperature range of carburizing (carbon nitrogen co carburizing) is 800-1050 ℃, based on the analysis of Ellingham oxygen potential diagram, the order of priority oxidation of different elements in this temperature range is C > Ce (rare earth element) ＞ Ba > mg > al > Ti > Si > b > V > NB > Mn > CR > C > Fe > P > Mo > Sn > Ni > as > Cu, as ＞ Cu, as shown in Figure 4.

Most common carburized steels contain alloying elements Cr, Mn, Ti and V (the most common is 20CrMnTi), which is easy to produce selective oxidation and non martensitic structure. Some heat treatment workers think that the internal oxidation tendency of Cr, Ti, V steel is small, and the higher hardenability can optimize the non martensitic structure. The author thinks that this is a misunderstanding, and may even mislead the mechanical design workers about the material selection of carburized steel.
According to the Ellingham oxygen potential diagram, Ni and Mo are not preferentially oxidized to C and Fe, so selective oxidation does not occur at the grain boundary, so intergranular oxidation (Igo) is not easy to occur. The selection of carburized steel containing Ni and Mo alloy elements is of positive significance for restraining non martensitic structure, such as 20CrNi2Mo and 17CrNiMo6.
Ellingham oxygen potential diagram is not omnipotent, but also has its limitations. The above-mentioned Ellingham oxygen potential diagram only represents the oxidation atmosphere derived from oxygen element. In addition to CO2, H2O, O2 and other oxidation components, there are also SO2, H2S and other oxidation substances, which should be fully considered in carburizing atmosphere control.

Improvement and optimization measures of non martensitic structure

Through the above analysis, it is not difficult to see that there are two main ways to solve the source of non martensitic structure: one is to minimize the elements that will be preferentially oxidized; The second is to reduce the oxidizing components of carburizing atmosphere (such as reducing oxygen partial pressure).
In the design and selection of carburizing steel, the content of alloying elements Cr, Mn, Ti and V should be controlled as much as possible on the premise of ensuring the hardenability of the material. At present, 20CrMnTi is the most common carburized gear in China, which tends to produce non martensitic structure, especially for some products with deep carburizing layer. It is suggested that mechanical designers should choose carefully. At present, a large number of materials such as 8620 (20CrNiMo) and 17CrNiMo6 are selected abroad, which are worthy of our reference.
It is very important to control the oxidizing components of carburizing atmosphere for improving non martensite:

• 1) Ensure the air tightness of carburizing equipment and prevent air from entering;
• 2) Strictly control the moisture of carburizing auxiliary materials, such as methanol, ammonia, propane, acetone, natural gas, etc;
• 3) Strictly control the organic sulfur, inorganic sulfur and other oxidizing components in carburizing auxiliary materials;
• 4) Increasing the amount of atmosphere replacement in the furnace, increasing the carbon potential properly, and introducing a certain amount of ammonia in the later stage of carburizing are all the methods to reduce the oxygen partial pressure in the furnace. Another good way to improve the non martensitic structure can be used for reference, which is rare earth catalytic infiltration technology. The author thinks that rare earth elements (such as cerium) have the characteristics of preferential selective oxidation to prevent the oxidation of effective alloying elements on the surface of the infiltrated layer. In a sense, rare earth elements can indirectly reduce the oxygen partial pressure.

When the non martensitic structure is unavoidable, it can be optimized to a certain extent by means of heat treatment process, quenching cooling method and strong shot peening, which will not be described here. The cooling characteristics of quenching oil also have a certain influence on non martensitic structure. Special attention should be paid to medium selection. The recommended selection principles are as follows:

• 1) On the premise of ensuring the hardness of effective hardening layer and core, the quenching oil with higher viscosity and higher initial boiling point should be selected as far as possible;
• 2) The characteristic temperature should be as high as possible to ensure the cooling rate of medium and high temperature on the surface of the infiltrated layer, and to shield the reduction of hardenability caused by selective oxidation.

Summary

This paper introduces the formation mechanism of non martensitic structure on the surface of carburized layer, and puts forward some measures for optimization and improvement. It is considered that “selective oxidation” is the essential reason for the formation of non martensite. As a basic tool of metallurgical physical chemistry, Ellingham oxygen potential diagram is also suitable for chemical heat treatment. The above views are expected to provide some reference for the basic research workers of materials.

Source: China Stainless Steel Flange Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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